This invention relates to resistance measurements and more particularly to an improved method and apparatus for measuring the operating resistance of an unknown resistance element to which repetitive high voltage pulses are applied.
Under certain applications, a resistance element may be subjected to repetitive high voltage pulses during operation. For example, radio frequency ignition noise suppression elements are commonly placed within the high voltage ignition circuit for a spark-ignited internal combustion engine. The suppression elements may be in the form of a distributed resistance or impedance in high voltage ignition cables or in the form of a lumped resistance or impedance element placed, for example, within the center electrode assembly of a spark plug. During operation of the engine, repetitive high voltage pulses of short duration are applied to the spark plugs in the engine for igniting a fuel-air mixture. Each high voltage pulse passes through an element which suppresses spurious emissions from the ignition system.
Heretofore, it has been difficult to accurately measure the resistance of an ignition noise suppression element under its operating conditions. A typical procedure for making such a measurement involved using an oscilloscope for measuring the peak voltage across and the peak current through the element while the element is subjected to high voltage pulses. The measured voltage is then divided by the measured current to obtain a measure of the effective operating resistance. Another method for measuring the resistance under high voltage conditions is with a resistance meter designed to apply a continuous high D.C. voltage to the unknown element and to determine the resistance from the current through the unknown element and the known voltage. Howevr, I.sup.2 R losses within the element are appreciably greater then when high voltage pulses are applied to the element. As a consequence, the element is heated to a greater than normal temperature, resulting in an erroneous resistance indication.